1. Field of the Invention
The present invention relates to a heater adapted for use in a heat fixing device to be mounted on an image forming apparatus utilizing electrophotographic or electrostatic recording method, such as a printer or a copying machine, and an image heating apparatus utilizing such heater, and more particularly to a heater having at least one cycle path of a heat generating resistor on a substrate and an image heating apparatus utilizing such heater.
2. Related Background Art
There will be explained an example in which a conventional heating apparatus is applied as an image heating apparatus (fixing apparatus) for heat fixing a toner image to a recording material, provided in an image forming apparatus such as a copying machine or a printer.
In an image forming apparatus, there has been widely employed a heating apparatus of heat roller type, as a fixing apparatus for heat fixing an unfixed image (toner image) of image information, which is formed in suitable image forming process means utilizing an electrophotographic process, an electrostatic recording process or a magnetic recording process, and borne on a recording material (transfer sheet, electrofax sheet, electrostatic recording paper, OHP sheet, printing paper, formatted paper etc.) by a transfer process or a direct process.
Recently, there is commercialized a heating apparatus of film heating type from the standpoint of quick starting or energy saving. The heating apparatus of such film heating type is proposed for example in Japanese Patent Application Laid-open Nos. 63-313182, 2-157878, 4-44075 and 4-204980.
In the heating apparatus of such film heating type, as shown in FIG. 12, a film (rotary member) 25 contains therein a heating member generally formed by a ceramic heater 20 (hereinafter also called a heater or a heating member), while a pressure roller 26 constituting another rotary member pressed to the film 25 is supported by an unrepresented support member, and the heater 20 and the rotary member 26 are pressed by pressurizing means (not shown) to form a pressed nip N. The heater 20 is composed of a heat-resistant base member 20a (hereinafter called heater substrate) and a heat generating resistance member 20b (also called resistor pattern) formed thereon by a thick film printing, and, on a sliding surface of the heater corresponding to the pressed nip N, there is provided a slidable member having a pressure resistance, a heat resistance and a low friction such as a glass coat layer 20c. 
FIGS. 13A and 13B show a position relationship of the heat generating resistor 20b in a plane of the heater 20. A heater shown in FIG. 13A has one cycle path (double path) of a heat generating resistor 20b on the heater substrate 20a. A forward path (forward side; ex. right side to left side) (half path) and a return path (return side; ex. left side to right side) (half pass) have a same resistance. Two current supply electrode patterns 20d, 20e are respectively connected electrically to ends of two heat generating resistors 20b of forward side and return side. A connecting electrode pattern 20f is provided for electrically connecting the other ends of the above-mentioned two heat generating resistors 20b of forward side and return side. Thus, the first current supply electrode pattern 20d, one (forward) heat generating resistor 20b, the connecting electrode pattern 20f, the other (return) heat generating resistor 20b and the second current supply electrode pattern 20e are electrically connected in series. An electric current is supplied between the first and second current supply electrode patterns 20d, 20e to generate heat from the two heat generating resistors 20b of forward side and return side.
Otherwise, the two heat generating members 20b of forward side and return side are given different resistances as shown in FIG. 13B to form a heat generating ratio between the upstream side and the downstream side, thereby varying heat distribution in the nip and optimizing the heat supply to the recording material.
Between such heater 20 and the pressure roller 26 constituting a pressurizing member, there is pinched a heat-resistant film 25 (also called a fixing film, or a fixing belt film) to constitute a pressurized nip N (also called a heating nip or a fixing nip), and the fixing film 25 and the pressure roller 26 are maintained in rotary motion. There are shown a rotating direction R25 of the fixing film 25, a rotating direction R26 of the pressure roller 26, and a conveying direction K of a recording material P.
Between the fixing film 25 and the pressure roller 26 in the pressed nip N, a recording material bearing an unfixed toner image to be fixed is introduced and conveyed together with the fixing film 25, whereby the heat of the ceramic heater 20 is given, in the pressed nip N, to the recording material P across the fixing film 25, and the unfixed toner image T is fixed to the recording material P by heat and pressure, under the pressure of the pressed nip N. In recent years, a further cost reduction is requested for the image forming apparatus including a copying machine and a printer. For such cost reduction, the size of the heater substrate 20a has been reduced thereby increasing the number of the heater substrates 20a obtained by cutting a single ceramic sheet, but the width of such substrate is now already reduced to several millimeters so that a further increase in the number of the heater substrates cut from a ceramic sheet does not contribute much to the cost reduction.
Also a smaller size of the heater substrate 20a decreases the nip N, whereby it becomes difficult to secure the fixing ability.
It is therefore conceivable, for securing the satisfactory fixing property even with a smaller width of the heater substrate, to increase an area of the heat generating resistors in the heater substrate as shown in FIGS. 13A and 13B, thereby effectively utilizing the size of the substrate.
However, in case the heat generating resistor is made wider (larger) as shown in FIGS. 13A and 13B, a resistance per a unit length becomes smaller for a same material of the heat generating resistor, whereby a designed resistance cannot be obtained in the entire heat generating resistor and the amount of heat generation becomes deficient. Consequently, in case of making the heat generating resistor wider, it is necessary to change a material constituting the heat generating resistor, in order to secure the resistance per unit length. The material for the heat generating resistor is principally constituted by silver and palladium (Ag/Pd), and a content of palladium has to be increased in order to increase the resistance. However, palladium is expensive, and an increase in the content thereof leads to a cost increase of the heater.
In consideration of the foregoing, an object of the present invention is to provide a heater having excellent heat generating characteristics even in a small size and an image heating apparatus utilizing such heater.
Another object of the present invention is to provide a heater of a low cost and an image heating apparatus utilizing such heater.
Still another object of the present invention is to provide a heater, including:
a substrate;
a heat generating resistor formed in at least a cycle path on the substrate; and
current supply electrodes provided at electrical ends of the heat generating resistor;
wherein a plurality of the heat generating resistors are connected in parallel to at least one of the current supply electrodes.
Still another object of the present invention is to provide an image heating apparatus including:
a heater, the heater including a substrate, a heat generating resistor formed in at least a cycle path on the substrate, and current supply electrodes provided at electrical ends of the heat generating resistor; and
a flexible sleeve rotating in a sliding contact with the heater;
wherein a plurality of the heat generating resistors are connected in parallel to at least one of the current supply electrodes.
Still another object of the present invention is to provide a heater, including:
a substrate;
a heat generating resistor formed on the substrate and including a serial connection of plural resistors of different resistances in at least two cycle paths.
Still another object of the present invention is to provide an image heating apparatus including:
a heater, the heater including a substrate, a heat generating resistor formed on the substrate and containing a serial connection of plural resistors of different resistances in at least two cycle paths, and current supply electrodes provided at electrical ends of the heat generating resistor; and
a flexible sleeve rotating in a sliding contact with the heater.
Still other objects of the present invention will become fully apparent from a following detailed description which is to be taken in conjunction with the accompanying drawings.